Process for reducing the sulphur content of anatase titania and the so-obtained product

ABSTRACT

The present invention relates to the field of heterogeneous catalysis. In more detail, it refers to a process for reducing the sulphur content of a stabilized titania, the so-obtained material and the use thereof for manufacturing of support materials for heterogeneous catalysts.

The present invention relates to the field of heterogeneous catalysis. In more detail, it refers to a process for reducing the sulphur content of stabilized anatase titania, the so-obtained catalytic support materials and the use thereof for manufacturing of heterogeneous catalysts.

Titanium dioxide is a well-known material for the manufacturing of heterogeneous catalysts. It finds widespread application either as the catalytic material (e.g. Claus catalysis) or as a catalytic support (e.g. selective catalytic reduction of nitrous oxides, Fischer-Tropsch).

The predominant and in most cases preferred polymorph for heterogeneous catalysis is the anatase crystal phase. The large industrial scale manufacturing of anatase type TiO₂ relies on the so-called sulphate process in which titanium rich raw materials (ilmenite or Ti-slag) are firstly reacted with concentrated sulphuric acid to form TiOSO₄. Upon hydrolysis, a fine particulate anatase type TiO₂ with a high water content is obtained (so-called metatitanic acid with general formula TiO(OH)₂). After further purification steps which include reduction and washing procedures, a pure anatase TiO₂ can be obtained.

The other large scale manufacturing process for TiO₂ is the so-called chloride process which uses a raw material with very high Ti content (natural or synthetic rutile or Ti-slag), chlorine and carbon to produce in a first step TiCl₄ which can easily be purified by distillation. Upon burning in an oxygen rich flame, a pure Rutile TiO₂ is obtained. A pure anatase TiO₂ polymorph cannot be produced by this method.

Another procedure for the manufacturing of anatase type TiO₂ is the flame hydrolysis of TiCl₄ yielding a mixture of Rutile and anatase only.

The performance of heterogeneous catalysts often depends on the purity. Stray ions can affect the overall conversion of the catalytic process and/or the selectivity. Typical unwanted impurities are phosphorous, sulphur, heavy metals, alkaline and alkaline earth metals.

For example, the Fischer-Tropsch synthesis of hydrocarbons from syngas (mixture of CO and H₂) is very sensitive towards sulphur impurities since the sulphur reacts with the catalytically active cobalt to form cobalt sulphides (Co_(x)S_(y)) which in turn lead to drastic reduced catalytic performance. Typical sulphur levels of FT-catalysts are below 150 ppm, preferably below 100 ppm. The major impurity in the sulphuric acid process generated anatase TiO₂ is sulphur stemming from adherent sulphuric acid of the manufacturing process. Other stray ion impurities are in the one or low two digit ppm range and typically are uncritical.

The performance of heterogeneous catalysts also depends on the physical properties. A very good dispersion of the catalytically active material on the support is often a prerequisite to observe high conversions. Typically large specific surface areas of the support are important to ensure maximum dispersion of the catalytically active centres.

As a summary, there is a need for large scale industrial availability of anatase type TiO₂ for catalytic applications that exhibits both

i) a large specific surface area (BET>40m2/g), and

ii) a low sulphur level (<150 ppm S).

From a manufacturing point of view, the solely large industrial scale and thus cost effective manufacturing process of anatase type TiO₂ is the sulphate process. Major drawbacks of this process is the large sulphur content in the final product which is known to be detrimental for a lot of catalytic applications. Thus, a process has to be found that allows for the large industrial scale production of an anatase type TiO₂ with high specific surface area (>40 m2/g) and a low amount of sulphur (<150 ppm S).

Several techniques have been developed to reduce the sulphur level in anatase type TiO₂ from the sulphate process. The most common one is the washing with water. Typically, the sulphate containing anatase TiO₂ is suspended in water and washed over a filter medium (e.g. filter press). The washing is performed with cold or preferably hot de-ionized water. The minimum sulphur levels that can be obtained by this process are in the range of 0.1-0.5 wt.-%.

Reacting the excess sulphuric acid with an appropriate base (NaOH, aqueous ammonia solution etc.) and removing the salts formed by excessive washing with de-ionized water allows for significant lower sulphur levels of 0.03-0.2 wt.-%. Especially when using basic solutions of metals (e.g. NaOH or KOH), a certain contamination risk exists, since using an excess amount of base in order to obtain lowest sulphate levels the metal ions are only hardly washed out of the anatase.

Lowering the sulphur level can be also be done by successive washing cycles by excess treatment with a strong base and successive removal of the metal ions by washing with an acid. In this case, it is preferred to use acids (e.g. acetic acid) that can easily be removed either during the washing or a potential subsequent heating step.

During manufacturing of pigmentary grade titanium dioxide, the sulphur is removed by thermal decomposition of the sulphuric acid. At temperatures exceeding 500° C. a significant reduction of the sulphate contaminations is observed, but during this heat treatment two processes also take place: i) the TiO₂ particle undergo a particle growth which results in significantly and irreversible decrease of the specific surface area and ii) at these temperatures the phase transformation from the anatase to the rutile polymorph takes place. Both processes are wanted in order to obtain pigmentary TiO₂ which typically is a low BET (<20m2/g) and Rutile type TiO₂, but they prevent this procedure from being used for large surface area, low sulphur anatase TiO₂ out of the sulphate manufacturing process.

As a consequence, there is no process available that allows for the production of an anatase type TiO₂ by a large industrial scale production that exhibits the following properties:

1. Ultra low sulphur content (<150 ppm).

2. BET surface area >20 m²/g, preferably >30 m²/g and more preferably >40 m²/g

3. TiO₂ in the pure anatase phase.

There is a need for a low sulphur anatase type catalytic support material with a high specific surface area that is easily accessible through large scale industrial processes.

In this context, it has surprisingly been found that anatase type titanium dioxide doped with the appropriate amount of silica and/or an oxide of zirconium, and or an oxide of aluminum can be treated at temperatures high enough to decompose the sulphuric acid while maintaining substantially large specific surface areas. In this context the term “thermal stabilization” has to be understood that anatase type TiO₂ is stabilized in a manner that i) the rutilization temperature is shifted towards higher temperatures and ii) the tendency towards BET loss is reduced.

In a typical experiment according to the invention, anatase type TiO₂ having a content of 8% wt % SiO₂ is heated for one hour to temperatures as high as 1000° C. The resulting powder exhibits BET surface areas of about 50-70 m²/g and residual sulphur contaminations of <50 ppm. The degree of resistance towards thermal aging of the anatase is strongly dependent on the amount of silica added. Small amounts only introduce a minor resistance, while larger amounts of silica have a strong effect on aging properties.

Besides this effect, silica can also influence the catalytic properties of the final catalyst. It can change the overall performance by altering the selectivity and/or the conversion rate. Depending on the specific application and its specific demands concerning BET surface area, SiO₂ and the residual S-content, the right material and calcination conditions have to be individually adjusted to the respective intended use. In general, high calcination temperatures reduce both, residual S-levels and specific surface area.

Basically, any element that is able to stabilise the anatase polymorph can be used in terms of this invention. Among numerous others typical elements for catalytic applications are Si, Al, Zr [J Mater Sci (2011) 46:855-874].

The incorporation of such stabilising elements can be achieved by a variety of different synthetic approaches. For the inventive material, the following different methods are suitable:

-   -   1. Precipitation of SiO₂ onto TiO₂     -   2. Co-precipitation or co-hydrolysis of TiO₂ and SiO₂     -   3. Mixing of TiO₂ sols and SiO₂ sols     -   4. Treating of TiO₂ with SiO₂ sols     -   5. Treating of TiO₂ with an SiO₂ precursor and subsequently form         SiO₂ via hydrolysis     -   and/or oxidation     -   6. Mixing TiO₂ and SiO₂.

Thus, the present invention is directed to an anatase titanium dioxide having a content of at least one compound selected from oxides of Si, Al, and Zr in an amount of 2-50% b.w., preferably 2-30% b.w., calculated as oxides, of the total weight of the oxides, and having a sulphur content of less than 150 ppm, preferably less than 100 ppm and more preferred of less than 80 ppm referred to the total weight of the oxides.

The inventive anatase material has preferably an alkali content such as of Na⁺ of below 200 ppm, preferably below 100 ppm in order to avoid any negative influences of the alkali on the stability of the material during use.

According to the invention, the anatase titanium dioxide is preferably obtained by the sulphate process which is obtained as titanium dioxide and hydrated forms thereof including meta-titanic acid. Meta-titanic acid and the hydrated forms of titania which are used here synonymously can be represented by the formula TiO_((2-x))(OH)_(2x) with 0≦x≦1, including also to titania. Said meta-titanic acid is then further treated to incorporate the stabilising agents selected from Si, Zr and/or Al in the form of the oxides and hydrated forms thereof and then subjected to the calcination treatment to decompose the sulphur-containing compound such as sulphuric acid as a remainder of the sulphate process. During calcination the hydrated forms are converted to the oxides and the hydrate content will be reduced to zero which should be clear to the skilled man.

The term “anatase titanium dioxide or anatase titania” as used in accordance with the present invention means that at least 95% b.w., preferably 98% b.w. and most preferred 100% of the titania is present in the anatase form. Generally, the anatase phase has crystallite sizes of 5-50 nm. Thus, for the inventive material, the crystalline phases of the particles are mostly present in the anatase phase, after drying at 105° C. for at least 120 min before calcination and also after calcination due to the stabilisation. I.e. after subtracting of the linear base, the ratio of the height of the most intensive peak of the anatase structure (reflex (101)) to the height of the most intensive peak of the rutile structure (reflex (110)) is at least 5:1, preferably at least 10:1. Most preferably, the XRD analysis exclusively shows anatase peaks. For determining the phase and crystallite size by Scherrer, in particular the crystal modification (phase identification), an X-ray is taken. For this, the intensities of the Bragg condition after diffracted at the lattice planes of a crystal X-rays are measured against the diffraction angle 2 Theta. The X-ray diffraction is characteristic for the phase.

Drying as used in the context of the present invention means drying at temperatures above 105° C. at ambient pressure. All large scale industrial techniques can be applied such as spin-flash or spray drying, but the drying is not limited to the techniques mentioned.

Calcining as used in accordance with the present invention means treating the stabilized anatase titania at an elevated temperature from above 500° C., preferably from 800° C. up to 1200° C., for a time period sufficient to decompose the remaining sulphur containing compound such as sulphuric acid and thus to reduce the sulphur content to a level below 150 ppm, preferably less than 100 ppm and more preferred less than 80 ppm referred to the total weight of the oxides, preferably for a time period of 30 min to 1200 min, while maintaining the titania in the anatase form. Calcining can be carried out in a regular calcination device under atmospheric pressure so that the sulphur containing components can evaporate from the material.

The weight ratios, ppm-values or percentages as used in the present invention refer to the weight of the material after calcination.

Due to the high temperature treatment, agglomeration can take place which can be detrimental for the subsequent processes for forming a catalyst. Thus de-agglomeration of the calcined material by milling can be necessary. Both, wet or dry milling techniques can be applied and typical techniques are ball or jet milling.

An optional sieving step to ensure removal of coarse particles can follow.

The anatase TiO₂ obtained can then serve as a catalytic support material which can further be treated with at least one compound of catalytically active metal selected from Co, Ni, Fe, W, V, Cr, Mo, Ce, Ag, Au, Pt, Pd, Ru, Rh, Cu, or mixtures thereof whereby a metal loaded material is obtained. A precursor compound soluble in polar or non-polar solvents of a catalytically active metal selected from Co, Ni, Fe, W, V, Cr, Mo, Ce, Ag, Au, Pt, Pd, Ru, Rh, Cu, or mixtures thereof can be used. Treating the support material with one precursor compound or mixtures thereof of the catalytically active metals can be performed by various techniques. Typical methods include incipient wetness or excess solvent method. Also deposition reactions such as hydrolysis can be applied to bring the catalytically active metal or precursors thereof into contact with the catalytic support material. The compound of a catalytically active metal which are not particularly limited and may be selected from Co, Ni, Fe, W, V, Cr, Mo, Ce, Ag, Au, Pt, Pd, Ru, Rh, Cu, or mixtures thereof can be used in an amount to obtain a loading of 1-50% b.w., preferably 5-30% b.w., and more preferably 8-20% b.w., calculated as oxides of the total weight of the final material.

Thus, the present invention covers an:

-   -   anatase titanium dioxide having a content of at least one         compound selected from oxides of Si, Al, and Zr in an amount of         2-50% b.w., preferably 2-30% b.w., calculated as oxides, of the         total weight of the oxides, and having a sulphur content of less         than 150 ppm, preferably less than 100 ppm and more preferred of         less than 80 ppm referred to the total weight of the oxides;     -   anatase titanium dioxide according to claim 1 having a content         of at least one compound selected from oxides of Si, Al, and Zr         in an amount of 3-20% b.w., more preferably 4-12% b.w.,         calculated as oxides, of the total weight of the oxides and         having a sulphur content of less than 150 ppm, preferably less         than 100 ppm and more preferred of less than 80 ppm referred to         the total weight of the oxides;     -   anatase titanium dioxide according to claim 1 having a content         of SiO₂ in an amount of 2-30% b.w., preferably 3-20% b.w., more         preferably 4-12% b.w., calculated as oxide, of the total weight         of the oxides, and having a sulphur content of less than 100         ppm, preferably less than 80 ppm referred to the total weight of         the oxides;     -   and a:     -   process for preparing the inventive anatase titanium dioxide         having a content of at least one compound selected from oxides         of Si, Al, and Zr in an amount of 2-50% b.w., preferably 2-30%         b.w., more preferably 3-20% b.w., most preferably 4-12% b.w.,         calculated as oxides, of the total weight of the oxides, and         having a sulphur content of less than 150 ppm, preferably less         than 100 ppm and more preferred less than 80 ppm, referred to         the total weight of the oxides, wherein:     -   a titanium compound selected from metatitanic acid or         titanylsulphate is mixed with at least one compound selected         from oxides and/or hydroxides of Si, Al, and Zr or precursors         thereof in an aqueous medium,     -   precipitating at least one compound selected from oxides and/or         hydroxides of Si, Al, and Zr,     -   treating the obtained product to reduce the alkali content         thereof if the alkali content is above 200 ppm, to a level of at         most 200 ppm, referred to the total weight of the oxides,     -   the product is optionally filtered, optionally washed with         water, and optionally dried, the product is then subjected to a         calcination treatment at a temperature of more than 500° C.,         preferably in the range of 800° to 1200° C., over a time period         sufficient to decompose the remaining sulphur containing         compound such as sulphuric acid to a level below 150 ppm,         preferably less than 100 ppm and more preferred less than 80 ppm         referred to the total weight of the oxides, preferably over a         time period of 0.5 to twelve hours,     -   process for preparing an embodiment of the inventive anatase         titanium dioxide wherein metatitanic acid is mixed with a SiO₂         precursor compound, precipitating at least one oxide and/or         hydroxide of Si, treating the obtained product to reduce the         alkali content thereof if the alkali content is above 200 ppm,         to a level of at most 200 ppm, referred to the total weight of         the oxides, optionally filtering, optionally washing the         obtained product and optionally drying the obtained product,         subjecting the product to a calcination treatment at a         temperature of more than 500° C., preferably in the range of         800° to 1200° C., over a time period sufficient to decompose the         remaining sulphur containing compound such as sulphuric acid to         a level below 100 ppm, preferably less than 80 ppm referred to         the total weight of the oxides, preferably over a time period of         0.5 to twelve hours,     -   Process for preparing an anatase titanium dioxide according to         claim 3 wherein a titanium compound selected from a TiO₂ sol is         mixed with an SiO₂ sol, adjusting the pH to obtain a         precipitate, treating the obtained precipitate to reduce the         alkali content if the alkali content is above 200 ppm referred         to the total weight of the oxides, to a level of at most 200         ppm, referred to the total weight of the oxides, the obtained         product is optionally filtered, optionally washed, optionally         dried, and the obtained product is subjected to a calcination         treatment at a temperature of more than 500° C., preferably in         the range of 800° to 1200° C., over a time period sufficient to         decompose the remaining sulphur containing compound such as         sulphuric acid to a level below 150 ppm, preferably less than         100 ppm and more preferred less than 80 ppm referred to the         total weight of the oxides, preferably in the range of 800° to         1200° C., preferably over a time period of 0.5 to twelve hours.     -   Process for reducing the sulphur content of a stabilised anatase         titania wherein an anatase titania having a content of a         stabilizing agent is treated at a temperature more than 500° C.,         preferably in the range of 800° to 1200° C., over a time period         sufficient to decompose a remaining sulphur containing compound         such as sulphuric acid to a level below 150 ppm, preferably less         than 100 ppm and more preferred less than 80 ppm referred to the         total weight of the oxides, preferably for a time period of at         least 30 min, wherein the stabilizing agent is selected from         oxides of Si, Al, and Zr and wherein the content of the         stabilizing agent is in the range of 2-50% b.w., preferably         2-30% b.w., calculated as oxides, of the total weight of the         oxides     -   Use of a calcination treatment at a temperature more than         500° C. for reducing the sulphur content of a stabilised anatase         titania having a content of at least one compound selected from         oxides of Si, Al, and Zr in an amount of 2-50% b.w., preferably         2-30% b.w., calculated as oxides, of the total weight of the         oxides, to a level below 150 ppm, preferably less than 100 ppm         and more preferred less than 80 ppm referred to the total weight         of the oxides.     -   Use of the anatase titanium dioxide of the invention, obtainable         according to the inventive processes, as a catalyst or catalyst         support in catalysis reactions, gas-to-liquid reactions such as         in particular Fischer-Tropsch catalysis, selective catalytic         reduction (SCR), oxidation catalysis, photo catalysis,         hydrotreating catalysis, Claus catalysis, phthalic acid         catalysis.     -   Catalyst or catalyst support, comprising the anatase titanium         dioxide of the invention, obtainable according to the inventive         processes.

The invention is further illustrated by the following Examples and Comparative Examples.

EXPERIMENTAL PART Analytical Methods Determination of TiO₂ Polymorph

In order to determine the TiO₂ polymorph, x-ray diffraction (XRD) analysis is applied. This is done in a typical XRD set-up where the intensities of the diffracted x-rays are measured vs. the diffraction angle 2 Theta. The evaluation of the xrd pattern is done using the JCPDS-data base. Typical condition of analysis are: 2 Theta=10°-70°, steps of 2 Theta=0.02°, measuring time per step: 1.2 s.

Determination of SiO₂ Content

The material is digested in H₂SO₄/(NH₄)₂SO₄, followed by dilution with de-ionized water.

The residue is washed with sulphuric acid and the SiO₂ content is obtained by weighing the filter cake after incineration.

Determination of TiO₂ content Digestion of the material is done with H₂SO₄/(NH₄)₂SO₄ or KHSO₄. Then reduction of the Ti⁴⁺ with Al to Ti³⁺ is done and finally the TiO₂ content is obtained by titration with ammonia iron-Ill-sulphate. (NH₄SCN as indicator)

Determination of S-Content

S-contents were obtained by elemental analyzer Euro EA (Hekatech). The sample is burned in oxygen atmosphere and the gases are analyzed by gas chromatography. S-contents are calculated from the areas of the chromatogram.

Determination of Specific Surface Area

The specific surface area was determined by nitrogen adsorption technique according to DIN ISO 9277 (BET method). 5 points between 0.1 and 0.3 p/p₀ were evaluated. The equipment used was an Autosorb 6 or 6B (Quantachrome GmbH).

EXAMPLE 1

SiO₂ (13.1% b.w.) was introduced by co-precipitation of TiO₂ and SiO₂ from TiOSO₄— and Na₂SiO₃-solutions. 352 l of Na₂SiO₃ (94 g/l SiO₂) solution and 2220 l of TiOSO₄ (103 g/l TiO₂) solution were simultaneously pumped over a period of 270 minutes into a stirred reaction vessel containing 960 l water. During the reaction, the pH was kept at 5 with ammonia solution. After the addition was complete, the reaction was heated for 1 hour to 75° C. to complete reaction. Afterwards a hydrothermal aging was performed for 4 hours at 9.5-10 bar and 170-180° C. Finally the resulting reaction mixtures was filtered and washed with de-ionized water. The product was obtained after spray drying at 350° C. BET was 100 m²/g and S content 4000 ppm.

Example 2

A SiO₂/TiO₂ powder having a SiO₂ content of 8.5% b.w. was prepared on the basis of metatitanic acid and Na₂SiO₃ following a sequence of pH-adjusting steps and final filtration and washing of the so-obtained material with de-ionized water. The SiO₂/TiO₂ powder obtained after drying had a BET of 334 m²/g and a sulphur content of 1100 mg/kg.

Example 3

943 g metatitanic acid (29.2% b.w. TiO₂) were diluted with deionized water to 150 g/L. 78.5 g ZrOCl₂×8H₂O were added and the temperature was raised to 50° C. Afterwards, 68 mL sodium silicate (Na₂SiO₃, 358 g/L SiO₂) were added. After addition was completed, aqueous NaOH (50% b.w. NaOH) was added until a pH of 5.25 at 50° C. was reached. The white precipitate was filtered and washed with deionized water until the conductivity of the filtrate was below 100 μS/cm. The remaining filter cake was dried at 105° C. BET-surface area of the product was 329 m²/g and S>1000 ppm. SiO₂ and ZrO₂ contents were 7.7% and 10.8% b.w. respectively.

Example 4

Example 4 was produced in the same way as example 3 except that the sequence of ZrOCl₂×8H₂O and sodium silicate addition was changed. For example 4 first the Na₂SiO₃ solution and afterwards the ZrOCl₂×8H₂O was added. SiO₂ and ZrO₂ contents were 6.8% and 10.4% b.w. respectively. BET-surface was 302 m²/g and S-content was 3300.

COMPARATIVE EXAMPLE 1

Hombikat 8602 (commercial product). BET surface area was 321 m²/g and S content 4700 ppm

COMPARATIVE EXAMPLE 2

Commercially available Hombikat 8602 was purified by neutralisation with NaOH and washing with deionized water. The resulting sulphur content before calcination was 0.2 wt.-% (2000 ppm). and BET-surface area 351 m²/g.

COMPARATIVE EXAMPLE 3

A rutile suspension was prepared according to example 1a in DE10333029A1. To this, NaOH was added to a pH of 6.0 to 6.2 at 60° C., the solid was filtered and washed with deionized water to a filtrate conductivity of below 100 μS/cm. The obtained filter cake was re-slurried and spray dried. The BET surface area was 105 m²/g and the S-content 70 ppm

COMPARATIVE EXAMPLE 4

Commercially available Aerosil P25 from Evonik was used as received. BET surface area was 55m2/g and S<30 ppm.

COMPARATIVE EXAMPLE 5

300 ml Titaniumxoychloride (145 g/L TiO₂) solution was diluted with de-ionized water to 3 L. Subsequently 4 g oxalic acid dihydrate were added and a white solid was deposited by treating the reaction mixture with aqueous 15% NaOH solution while maintaining the temperature below 20° C. The final pH was 6.2. After filtration the white solid was washed with de-ionized water to a filtrate conductivity <100 μS/cm. Re-slurrying and spray drying gave the final product with BET: 359 m²/g and S<30 ppm.

Calcination

All calcinations were conducted in a muffle kiln. The materials were placed into ceramic seggars (corundum) and heated for 1 hour at 1000° C. The resulting powders were carefully grinded and homogenised prior to XRD, BET and SO₄ analyses. The BET surface areas and sulphur contents of various SiO₂-treated TiO₂ anatase supports before and after aging for 1 h at 1000° C. are shown in Table 1.

Fischer Tropsch Synthesis (FTS):

The FTS test were conducted using a 32-fold parallel reactor. The powders were compacted and subsequently crushed. The samples were lowed with Co(NO3)2 via impregnation in order to get a final Co loading of 10 wt.-% based on the total weight of the dried and reduced catalyst. For catalytic testing, the 125-160 μm fraction was used and each catalyst unit was filled with an amount of catalyst to ensure 40 mg Co-metal loading. Prior to the catalytic testing the catalyst was activated in diluted H₂ (25% in Ar) at 350° C. (1K/m in heating ramp). The catalytic testing was then performed at 20 bar with a feed of 1.56 L/h per reactor. The H₂/CO ratio was 2 (10% Ar in feed) and the temperature of the catalytic test was 220° C.

In Fischer Tropsch synthesis, CO and H₂ are contacted at elevated pressure and temperature to react to hydrocarbons. Evonik P25 is a known TiO₂ based catalytic support for this application. In order to have an overall economic FTS process, the catalysts have to fulfil the properties:

1. High CO conversion (X_(CO) in %)

2. High C₅₊ productivity (P_(C) ₅₊ in g_(C) ₅₊ /(g_(Co)h))

3. Low methan selectivity (S_(CH) ₄ in %)

4. Low CO₂ selectivity (S_(CO) ₂ in %)

The target of FTS is to produce long chain hydrocarbons. Especially hydrocarbons with more than 5 carbon atoms are of interest, because they serve as a feedstock e.g. for high quality Diesel, kerosene or long chain waxes. Syngas (H₂/CO-mixtures) is often produced from methane by reacting it with H₂O to yield CO and H₂ (steam reforming). The reverse reaction would reduce the amount of CO and H₂ available for the FTS reaction. High CH₄ selectivity in FTS indicates high conversion of CO and H₂ to CH₄ and vice versa. Therefore the CH₄ selectivity should be kept at lowest level possible. Additionally under the reaction conditions CO can react with H₂O to form CO₂ and H₂ (water gas shift reaction). This would reduce the concentration of carbon atoms available for the FTS. High CO₂ selectivity indicates high conversion of CO to CO₂ and vice versa. Thus CO₂ selectivity should be low for FTS catalysts.

Besides this, CO conversion (the amount of CO converted) should be high and additionally the amount of hydrocarbons with more than 5 carbon atoms should also be high. The latter parameter is indicated by the amount of hydrocarbons with more than 5 carbon atoms produced within one hour over one gram of Cobalt metal.

With respect to all these four parameters, Table 3 clearly shows that the inventive products exhibit superior properties when used as catalytic supports in FTS.

TABLE 1 Post calcination Pre calcination at 1000° C. for 1 h BET S TiO2 - BET S TiO2 Sample m²/g mg/kg Polymorph m²/g mg/kg Polymorph Example 1 100 4000 Anatase 60 40 Anatase Example 2 334 1100 Anatase 70 <30 Anatase Example 3 329 >1000 Anatase 77 <30 Anatase Example 4 302 3300 Anatase 52 <30 Anatase Compar- 321 4700 Anatase 3 <30 Rutile ative Example 1 Compar- 351 2000 Anatase 3 <30 Rutile ative Example 2

TABLE 2 Analysis overview of support materials used for FTS BET S m²/g mg/kg TiO₂ Polymorph Example 2 (after 1 h 1000° C.) 70 <30 Anatase Example 3 (after 1 h 1000° C.) 77 <30 Anatase Example 4 (after 1 h 1000° C.) 52 <30 Anatase Comparative Example 3 105 70 Rutile Comparative Example 4 55 <30 Anatase/Rutile Comparative Example 5 359 <30 Anatase

TABLE 3 Fischer Tropsch synthesis data of Inventive and Comparative Examples P_(C) ₅₊ X_(CO) % S_(CH) ₄ % g_(C) ₅₊ /(g_(Co)h) S_(CO) ₂ % Example 2 54 7.2 3.46 0.6 Example 3 55.2 7.8 3.35 0.7 Example 4 52.9 7.7 3.3 0.6 Comparative Example 3 12.6 9.4 0.74 n.d. Comparative Example 4 20.6 9.5 1.18 n.d. Comparative Example 5 0.5 31.3 0.02 n.d. n.d. = not determined because CO conversion was too low.

The above results of the Examples according to the invention and of the Comparative Examples as well as the catalytic tests show that the combination of the properties of the inventive materials, i.e. high specific surface area, anatase content and low sulphur content lead to superior catalytic properties thereof. 

1. Anatase titanium dioxide having a content of at least one compound selected from oxides of Si, Al, and Zr in an amount of 2-50% b.w., preferably 2-30% b.w., calculated as oxides, of the total weight of the oxides, and having a sulphur content of less than 150 ppm, preferably less than 100 ppm and more preferred of less than 80 ppm referred to the total weight of the oxides.
 2. Anatase titanium dioxide according to claim 1 having a content of at least one compound selected from oxides of Si, Al, and Zr in an amount of 3-20% b.w., more preferably 4-12% b.w., calculated as oxides, of the total weight of the oxides and having a sulphur content of less than 150 ppm, preferably less than 100 ppm and more preferred of less than 80 ppm referred to the total weight of the oxides.
 3. Anatase titanium dioxide according to claim 1 having a content of SiO₂ in an amount of 2-30% b.w., preferably 3-20% b.w., more preferably 4-12% b.w., calculated as oxide, of the total weight of the oxides, and having a sulphur content of less than 100 ppm, preferably less than 80 ppm referred to the total weight of the oxides.
 4. Process for preparing an anatase titanium dioxide having a content of at least one compound selected from oxides of Si, Al, and Zr in an amount of 2-30% b.w., preferably 3-20% b.w., more preferably 4-12% b.w., calculated as oxides, of the total weight of the oxides, and having a sulphur content of less than 150 ppm, preferably less than 100 ppm and more preferred less than 80 ppm, referred to the total weight of the oxides, of claim 1 wherein: a titanium compound selected from metatitanic acid or titanylsulphate is mixed with at least one compound selected from oxides and/or hydroxides of Si, Al, and Zr or precursors thereof in an aqueous medium, precipitating at least one compound selected from oxides and/or hydroxides of Si, Al, and Zr, treating the obtained product to reduce the alkali content if the alkali content is above 200 ppm, to a level of at most 200 ppm, referred to the total weight of the oxides, the product is optionally filtered, optionally washed with water, and optionally dried, the product is then subjected to a calcination treatment at a temperature of more than 500° C., preferably in the range of 800° to 1200° C., over a time period sufficient to decompose the remaining sulphur containing compound such as sulphuric acid to a level below 150 ppm, preferably less than 100 ppm and more preferred less than 80 ppm referred to the total weight of the oxides, preferably over a time period of 0.5 to twelve hours.
 5. Process for preparing an anatase titanium dioxide according to claim 3 wherein metatitanic acid is mixed with a SiO₂ precursor compound, precipitating at least one oxide and/or hydroxide of Si, treating the obtained product to reduce the alkali content if the alkali content is above 200 ppm, to a level of at most 200 ppm, referred to the total weight of the oxides, optionally filtering, optionally washing the obtained product and optionally drying the obtained product, then subjecting the product to a calcination treatment at a temperature of more than 500° C., preferably in the range of 800° to 1200° C., over a time period sufficient to decompose the remaining sulphur containing compound such as sulphuric acid to a level below 100 ppm, preferably less than 80 ppm referred to the total weight of the oxides, preferably over a time period of 0.5 to twelve hours,
 6. Process for preparing an anatase titanium dioxide according to claim 3 wherein a titanium compound selected from a TiO₂ sol is mixed with an SiO₂ sol, adjusting the pH to obtain a precipitate, treating the obtained precipitate to reduce the alkali content if the alkali content is above 200 ppm referred to the total weight of the oxides, to a level of at most 200 ppm, referred to the total weight of the oxides, the obtained product is optionally filtered, optionally washed, optionally dried, and the obtained product is subjected to a calcination treatment at a temperature of more than 500° C., preferably in the range of 800° to 1200° C., over a time period sufficient to decompose the remaining sulphur containing compound such as sulphuric acid to a level below 150 ppm, preferably less than 100 ppm and more preferred less than 80 ppm referred to the total weight of the oxides, preferably in the range of 800° to 1200° C., preferably over a time period of 0.5 to twelve hours.
 7. Process for reducing the sulphur content of a stabilised anatase titania wherein an anatase titania having a content of a stabilizing agent is treated at a temperature more than 500° C., preferably in the range of 800° to 1200° C., over a time period sufficient to decompose a remaining sulphur containing compound such as sulphuric acid to a level below 150 ppm, preferably less than 100 ppm and more preferred less than 80 ppm referred to the total weight of the oxides, preferably for a time period of at least 30 min, wherein the stabilizing agent is selected from oxides of Si, Al, and Zr and wherein the content of the stabilizing agent is in the range of 2-50% b.w., preferably 2-30% b.w., calculated as oxides, of the total weight of the oxides.
 8. Use of a calcination treatment at a temperature more than 500° C. for reducing the sulphur content of a stabilised anatase titania to a level below 150 ppm, preferably less than 100 ppm and more preferred less than 80 ppm referred to the total weight of the oxides.
 9. Use of the anatase titanium dioxide according to claim 1 or obtainable according to the process of claim 4, as a catalyst or catalyst support in catalysis reactions, gas to liquid reactions such as in particular Fischer-Tropsch catalysis, selective catalytic reduction (SCR), oxidation catalysis, photo catalysis, hydrotreating catalysis, Claus catalysis, phthalic acid catalysis.
 10. Catalyst or catalyst support, comprising the anatase titanium dioxide according to claim 1 or obtainable according to the process of claim
 4. 